1. Field of the Invention
The present invention relates generally to satellite communications, and more particularly to cell hopping in a satellite communications system.
2. Related Art
The current trend in cellular satellite communication systems is to divide the coverage space into hexagonal cells and assign cell frequencies based on cell separation. With a non-hopped multi-beam antenna system, each cell is then serviced by one staring beam (or two beams if the cell is dual polarized). The maximum capacity of the busiest cell is therefore limited by the capacity of one beam (or of two beams if the cell is dual polarized). Furthermore, the total number of covered cells is limited by the number of beams in the antenna system.
The cell size depends on the characteristics of the antenna, and is selected to simultaneously optimize the coverage region, the maximum coverage capacity, the signal power, and the signal to interference ratio (C/I) of each cell. A common solution is to use a tapered antenna to reduce the sidelobe (or co-channel) interference, and make the cell size the 3 to 5 dB beamwidth of the antenna. This provides a compromise between signal integrity and total coverage area.
The present invention is an apparatus, method, and computer program product for assigning communication resources in a beam-hopping cellular communication system. The satellite has a multiple beam antenna that covers a number of cells that is greater than the number of available beams. In a preferred embodiment, the method includes the steps of selecting a frequency for each beam, computing a dwell time for each cell based on the traffic estimates for each cell and the number of available beams, and selecting a cell hopping sequence for each beam based on the dwell times and predicted inter-beam interference.
Preferably the method also includes computing a dwell time percentage for each cell as the ratio of the traffic estimate for the cell to the total traffic estimate for all cells.
Preferably the method also includes defining a coverage template, the coverage template having a row for each beam and a column for each of a predetermined number of uniform time slots, and allocating a number of the time slots to each cell, the number based on the dwell time percentage for the cell. The coverage template is used to control the hopping sequence of each beam.
Preferably the method also includes selecting the cell that is not assigned to a beam and is allocated the greatest number of time slots, selecting the beam having the greatest number of unassigned time slots, assigning the selected cell to the selected beam, and repeating these steps until all of the cells are assigned to beams.
Preferably the method also includes predicting the inter-beam interference that would result from the assignment, and assigning the cell to the beam having the next greatest number of unassigned time slots when the predicted inter-beam interference exceeds a predetermined threshold.
Preferably the method also includes reducing the time slot allocation of each cell, and initializing the coverage template and re-assigning cells to beams, when a cell cannot be assigned to a beam. The time slot allocation for each cell is reduced according to its dwell time percentage.
Preferably the method also includes increasing the time slot allocation of each cell when unassigned time slots remain in the coverage template for the beam after all cells have been assigned. The time slot allocation for each cell is increased according to its dwell time percentage when the predicted inter-beam interference is below a further predetermined threshold.
Preferably the method also includes predicting the inter-beam interference based on the antenna pattern of the multiple beam antenna.
Preferably the method also includes updating the traffic estimates, modifying the dwell time for each cell based on the updated traffic estimates for each cell and the number of available beams, and modifying the cell hopping sequence for each beam based on the modified dwell times.
One advantage of the present invention is that it increases the capacity (and hence the revenue) of cellular satellite communication systems using a new technique that combines a hopped multi-beam antenna with a frequency/time assignment scheme that is adaptable to the traffic demands of the coverage cells.
Another advantage of the present invention is that it simultaneously increases the signal to interference ratio and the capacity of each cell in the coverage area.